1. Field of the Invention
The present invention relates to an improved zoom lens system for use as a zoom lens in a video camera recorder, a digital still video camera or the like. More particularly, the present invention relates to a compact zoom lens system for varying a focal length and providing improved magnification and back focus.
2. Description of Related Art
In conventional zoom lens systems, most wide angle zoom lenses use a "two-lens group" system which are typically provided in single lens reflex film cameras which do not use a color filter or Charge Coupled Device (CCD) as is used in a video camera recorder or a digital still video camera.
A typical lens system for use in a conventional film camera and not adapted for use in a camera having a color filter or CCD is described in Japanese Koukoku Patent No. 3-20735. The zoom lens system of this prior art reference differs from the conventional two lens-group system by inserting a lens unit having positive refractive power at a space located between a first lens group having a negative refractive power and a second lens group having a positive refractive power, described from the object side, so as to provide three "lens groups." The lens system of this reference also provides desired spacing between the first lens group and the lens unit and between the second lens group and the lens unit to provide for more effective compensation of distortion, to miniaturize the lens system and to decrease a minimum object distance during focus adjustment. This zoom lens system performs zooming by displacing the first and second lens groups and the one lens unit independently while maintaining the position of the image surface constant.
With such a device, it is not required or desirable to have an image surface located or moved far from an exit pupil but instead, the image surface is preferably located close to the exit pupil. Further, the lens unit or third lens group is located between the first and second lens groups.
As is mentioned above, the zoom lens system of JP 3-20735 is not adapted for use with a color filter or CCD as is required in a video camera recorder or a digital still video camera. Designing a zoom lens system for use with a color filter and CCD presents special problems. In a typical device, a lens is spaced from a color filter, such as an infrared cut filter which may include low-pass filters disposed on either side of the infrared cut filter, and the color filter is spaced away from a CCD. The lens, the color filter and the CCD are aligned along an axis.
As is well known, the color filter includes a plurality of pixels which are aligned with a plurality of pixels contained in the CCD to ensure that an image passing through the lens, the color filter and the CCD is correctly formed without distortion and other defects. When any portion of a light flux passing through the lens to the color filter and then to the CCD is inclined relative to the optical axis, an image cannot be formed correctly and the image is distorted resulting in a poor telecentric character. Thus, when a light flux is inclined relative to the optical axis, a telecentric character is poor.
It was discovered that a poor telecentric character can be overcome and a good telecentric character can be provided if an exit pupil is located sufficiently far from an image surface. Thus, when a zoom lens system is arranged such that an exit pupil is located far from an image surface, the telecentric character is good.
Japanese Laid Open Patent Number 6-94996 teaches such a conventional zoom lens system for use with a camera having a color filter and CCD which provides a good telecentric character by arranging an exit pupil far from an image surface. The structure of the device described in JP 6-94996 is shown in FIG. 31 of the present application. As seen in FIG. 31, the zoom lens system of JP 6-94996, includes successively from an object side, a first lens group I having a negative refracting power, a second lens group II having a positive refracting power and a third lens group III having a positive refracting power. In this system, the first lens group I moves with the second lens group II as shown in FIG. 31 to correct the movement of the focus position according to the variable power. The third lens group III remains stationary during power variation. An aperture diaphragm is fixed between the first lens group I and the second lens group II.
Although the zoom lens system shown in FIG. 31 solves the problems described above and provides an acceptable telecentric character, the zoom lens system of FIG. 31 is only capable of achieving a relatively small magnification and back focus. For example, it has been determined that the lens system shown in FIG. 31 can only achieve a magnification ratio of 1.9 and a back focus of 6.1. Thus, the zoom lens system of FIG. 31 has serious disadvantages for use in a video camera recorder, a digital still video camera or the like because of the severely limited magnification ratio and back focus.